The American Heart Association guidelines for the correct application of cardio-pulmonary resuscitation (CPR) specify that chest compressions be performed at the rate of 80 to 100 per minute and at a depth, relative to the spine, of 1.5 to 2.0 inches. (Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care, 102 Circulation Supp. I (2000).) However, CPR is physically and emotionally challenging, even for trained professionals. Research has shown that manual chest compressions rarely meet the guidelines. See, for example, Ochoa et al., The Effect of Rescuer Fatigue on the Quality of Chest Compressions, Resuscitation, vol. 37, p.149–52. See also Hightower et al., Decay in Quality of Closed-Chest Compressions over Time, Ann Emerg Med, 26(3):300–333, September 1995. One of the difficulties of performing correct chest compressions is that the rescuer imprecisely judges the timing and depth of compressions, particularly when the rescuer becomes tired. Thus, if accurate and timely user feedback could be provided to the rescuer then the rescuer would be more likely to perform CPR correctly.
Various devices have been proposed to assist a rescuer in properly applying CPR. For example, Kelley, Apparatus for Assisting in the Application of Cardiopulmonary Resuscitation, U.S. Pat. No. 5,496,257 (Mar. 5, 1996) shows a device that uses a pressure sensor to monitor compression forces and timing. Groenke et al., AED with Force Sensor, U.S. Pat. No. 6,125,299 (Sep. 26, 2000) shows a device that uses a force sensor to measure the compression force applied to a patient's chest. However, these devices only measure the force applied to the chest and do not measure the actual depth of compressions. A given force can compress the chests of different patients by different amounts, so measuring only force will not provide sufficient or consistent feedback to the rescuer. In addition, force-based measurements may also be inaccurate because of intra-patient variation in thoracic morphology and compliance (stiffness).
CPR devices that use only accelerometers to measure depth of compressions, other than our own patented device shown in Halperin et al., CPR Chest Compression Monitor, U.S. Pat. No. 6,390,996 (May 21, 2002), do not fully or accurately account for errors in the measured acceleration; nor do they account for drift in the starting points of compressions. In addition, the integration process necessary to derive the depth of compressions greatly compounds any errors in the measured acceleration.
It is important to correct for errors in the measured acceleration since the total depth of compressions should be within the relatively narrow range of 1.5 inches to 2.0 inches. Numerical simulations have shown that a total error in acceleration as small as 0.02 in/sec2 results in an error of 0.25 inches in displacement. Given the narrow depth range of optimal compressions, an error of 0.25 inches is unacceptable. For example, Freeman, Integrated Resuscitation, U.S. Publication 2001/0047140 (Nov. 29, 2001) shows a device that uses an accelerometer as a compression sensor and mentions gauging chest depth with the accelerometer. However, Freeman enables no method to account for the errors inherent in using an accelerometer alone. Thus any measurement Freeman makes of chest compression depth is inaccurate.
Myklebust et al., System for Measuring and Using Parameters During Chest Compression in a Life-Saving Situation or a Practice Situation and Also Application Thereof, U.S. Pat. No. 6,306,107 (Oct. 23, 2001) describes a device which uses a pressure pad, containing an accelerometer and a force activated switch, to determine the depth of depressions. However, Myklebust does not provide a means to measure compression depth using an accelerometer alone, nor does Myklebust account for some kinds of error in the measured value of chest compression depth (such as drift).
The problems inherent in the above devices show the difficulty of solving the problem of measuring chest compression depth using only an accelerometer. Nevertheless, the basic concept of determining displacement from a measured acceleration is straightforward (in a system with a known starting position). Displacement is determined by double integrating the measured acceleration.
However, this method of measuring chest compression depth is complicated by at least three major sources of error: signal error, external acceleration error, and drift in the actual or measured starting points of compressions from the initial starting point of compressions. Signal error comprises errors in the measured acceleration due to electronic noise, the shaking of wires or cables, errors inherent in the accelerometer, and other sources of noise in the acceleration itself.
External acceleration error comprises errors introduced by accelerations applied to the patient and/or the accelerometer other than accelerations caused by CPR. For example, if the patient is being transported in an ambulance and a rescuer is applying manual CPR with a compression monitor, then the accelerometer will measure accelerations caused by road vibrations as well as accelerations caused by CPR. (If the ambulance hits a pot hole then a large spike may appear in the compression waveform.) The accelerometer, by itself, cannot distinguish between the accelerations caused by road noise and the accelerations caused by compressions. In other words, the accelerometer measures a combined acceleration and not just the accelerations caused by compressions. Accordingly, the compression monitor will report a displacement value different from the actual chest displacement.
Another source of error, drift, comprises systematic shifts in the actual or reported starting points of each compression over an entire series of compressions. The accelerometer has no “memory” of the initial starting position. Thus, as the rescuer applies compressions the reported depth waveform can start to drift. The compression monitor may indicate that the reported depth waveform is increasingly deeper than the actual waveform. This form of drift is referred to as positive drift. On the other hand, drift can also cause the compression monitor to report a depth waveform that is increasingly more shallow than the actual waveform. In other words, actual compression starting points are becoming increasingly deeper, but the compression monitor instead reports each starting point as close to the initial starting point. This form of drift is referred to as negative drift.
One cause of negative drift is a failure to allow the chest to return to a fully relaxed position. Absent correction, the accelerometer will begin measuring displacement from the new “initial” position. Thus, the compression monitor erroneously informs the rescuer that the current starting point is at the initial starting point. However, the actual depth of the current starting point is more than the depth reported by the compression monitor. As a result, the rescuer may compress the chest harder than he should to achieve the erroneous depth suggested by the compression monitor.
Another source of both types of drift is a change in the overall position of the accelerometer with respect to the patient. For example, if the accelerometer is not fully secured then the accelerometer may systematically slip. (This may also cause external acceleration error.) Yet another source of drift is expansion and contraction of the chest due to ventilation performed simultaneously with compressions. Other sources of drift may also exist. Each source of drift may be independent of the others and may not cancel each other out, so the compression monitor should be able to account for both positive and negative drift.
Notwithstanding drift resulting from erroneous operation, changes in the actual starting point of compressions do occur. For example, if one or more ribs break during CPR then the actual starting point of each compression may be closer to the spine (a phenomena known as chest remodeling). Other types of chest injury or disease that affect the structure and strength of the rib cage can also cause chest remodeling. Chest remodeling can be gradual, in which case a gradual shift occurs in the actual initial starting point of compressions. A compression monitor should be able to account for the difference between erroneous drift and an actual shift in the starting points of compressions.
These and other sources of error are compounded by integrating the acceleration. The errors caused by signal noise and drift cause the constants of integration to have a value other than zero. The non-zero constants of integration compound the errors already present in the acceleration. Thus, the total compression depth reported by the compression monitor can be very inaccurate. Accordingly, methods are needed to accurately and precisely derive the depth of chest compressions from a measured acceleration.